There exist various kinds of malfunctions of electronic devices under usage conditions thereof. Above all, a malfunction of a joint portion such as a solder joint is particularly one of troublesome defect phenomena which occur frequently. Once such a malfunction occurs, the malfunction causes a serious influence to device operation. Even a small strain fracturing nothing at one time could accumulate at a solder joint as a result of repetition of various loads, e.g., thermal loads due to power ON/OFF or external mechanical loads, thereby causing metallic fatigue at the solder joint. There is known a structural health monitoring technology of an electronic device to predict a life of the electronic device until the electronic device breaks down because of solder joint trouble due to such a fatigue phenomenon.
In order to accurately predict a metallic fatigue life for solder joints or other parts metals, it is important to accurately estimate an amount of strain occurring at a point to be estimated. However, in most cases, variations in the strain involved in a damage progression such as a crack progression at a solder joint are not estimated in the life prediction of a BGA (Ball Grid Array) solder joint. The variations in the strain amount involved in the crack progression at the solder joint are not taken into consideration for the life prediction of the BGA solder joint. That is, in most cases, a strain estimation is carried out as is for an initial damage progression at a solder joint even for the damage progression subsequent to the initial one, i.e., even after the strain amount becomes large as a result of a decrease in stiffness at the solder joint. Here is a time-consuming job to prepare a damage model (i.e., a model to estimate damage indexes of a solder joint on the basis of a temperature change) and simpler algorithm for implementation. When such a time-consuming job is taken into consideration, a method without involving the time-consuming job may provide a practical prediction even though the life prediction accuracy thereof lowers. However, under normal conditions, strain amplitude accumulated at the solder joint promotes cracks in the solder to decrease stiffness of the solder joint. Therefore, a rate of stress to be received by each solder joint varies in accordance with temperature change. Accordingly, a life of a signal bump at an inner location which is predicted from a dummy bump at an outer location is to cause an error against an actual life unless progression of damage is considered. Here, the error of life is mainly caused by a difference between estimated strain and actual strain of each bump due to stiffness variations.